Computers (also referred to as “computing devices”) use a communications or data bus that has two or more lines to transfer electrical communication signals to another external device. A currently popular computer peripheral bus is the well-known universal serial bus (USB), which is used in a wide range of computing devices. One of the desirable features of a USB connector design is that in addition to the communication contacts, there are one or more power supply contacts on which power can be provided from one device to the other connected device. This allows the bus connector to be used not just for communications but also for charging the battery of a portable device from, for example, an external power source such as an AC wall power adapter, an automotive power adapter, or another computing device such as a desktop computer.
If the specified power supply voltage on the power supply contact of a bus connector is much higher than the allowed signal swing on the communications lines, then this presents a problem during an accidental short circuit situation in which a low impedance electrical path is accidentally formed between the communication contact and the power supply contact. That is because an input/output (I/O) port of a sensitive electronic circuit, such as that of a digital microelectronic integrated circuit, e.g. a microcontroller or a microprocessor, that is connected to the communications contact, could be damaged by the over voltage condition that is created by the external short circuit. One approach to alleviating the over voltage stress that may be inflicted upon the sensitive electronic circuit is to add a clamping circuit that is connected between the communications contact and the power supply of the microelectronic integrated circuit. The clamping circuit automatically limits the excursion of the voltage on the communications contact to only one diode drop above the power supply voltage of the sensitive electronic circuit. Such clamping circuits, however, form a so-called parallel connection to the communications contact and as a result increase the parallel capacitance on that contact, thereby degrading the maximum speed of the communications through that contact.
A protection circuit has been suggested that helps prevent the above-described over voltage condition on the communications contact, while not significantly degrading communications speed. In that technique, a variable resistance device is connected between the sensitive integrated circuit port and the connector contact. As an example, a metal oxide semiconductor field effect transistor (MOSFET) may be used, that is controlled by an over voltage detector such that during an over voltage event, the detector turns off the transistor (open circuit state). The gate terminal of the transistor is used as a threshold detector, by connecting the gate to the power supply contact Vbus of the connector. If the voltage on the communication line does not exceed one transistor threshold drop below the power supply contact voltage Vbus, then the transistor stays on (short circuit state), which allows the communications signal to be transmitted through. If, however, the voltage on the communications contact reaches the power supply contact voltage Vbus, then the transistor begins to switch into its open circuit state, presenting a high series resistance between the communications contact and the integrated circuit port, thereby preventing the voltage on the integrated circuit port from rising above Vbus. This should help protect the integrated circuit port from seeing voltages higher than Vbus that might otherwise damage the port.